1. Field of Invention
The present invention relates to nanosized silver oxide powder and a method of making the same.
2. Description of Related Art
Powdered silver oxide (Ag2O) is used in a variety of applications such as, for example, in the formulation of ink compositions for forming conductive films in electronic devices, as a reactant or catalyst in chemical reactions and in the formulation of antimicrobial compositions. In many applications, it is advantageous for the powdered silver oxide to have an extremely small particle size. Smaller particles have more surface area than larger particles per unit of mass, which means that smaller particles tend to exhibit enhanced reactivity as compared to larger particles. Moreover, under certain circumstances, silver oxide having an extremely small particle size can be reduced to silver metal at temperatures below 100° C., which can be advantageous in particular applications such as in the formation of electrically conductive thin films.
A common method of producing silver oxide powder is to react an aqueous silver nitrate solution with an aqueous sodium hydroxide solution to precipitate silver oxide according to the following reaction:2AgNO3+2NaOH═Ag2O↓+2NaNO3+H2O
This reaction, when run at a 63.4% AgNO3 (40.3% Ag by weight) concentration by weight in H2O and 50% NaOH concentration by weight in H2O at 100° C., is known to precipitate silver oxide particles that have an average diameter (also sometimes referred to as “particle size”) of about 2 μm. Asada et al., U.S. Pat. No. 4,080,210, disclose a method and apparatus for preparing silver oxide particles that utilizes the same chemical reaction, but at different reaction conditions, which produces silver oxide particles having an average particles size of about 0.3 to 0.6 μm.
By decreasing the concentration of the reactants to 5% and by running the chemical reaction at a temperature of about 22.5° C., applicants were able to precipitate silver oxide having an initial average particle size of about 150–200 nm, as determined via measurements made using a scanning electron micrograph (SEM). Unfortunately, the 150–200 nm precipitated silver oxide particles were unstable, meaning that the particle size was observed to increase over a relatively short period of time until the particles exhibited an average particle size of about 0.3 to about 1.8 μm, as determined by SEM measurements.
The undesirable increase in particle size of conventional nanosized silver oxide powder is most likely attributable to Ostwald ripening, which states that in a system of pure crystals of various sizes under a mother liquor, smaller crystals will tend to dissolve and re-precipitate out on the surface of larger crystals thereby increasing the average crystal size of the product. Investigation of the growth of nanosized silver oxide particles suggests that the Ostwald ripening effect is most significant during the initial precipitation step prior to filtration and during water washing.